1. Field of the Invention
The present invention relates to circuitry utilized to generate microwave signals at relatively high power levels. Particularly, the invention relates to frequency multiplier circuitry for increasing the output frequency of a power signal provided by an amplifier operated in the microwave range.
2. Description of the Prior Art
It is very difficult to directly generate relatively high power output signals at higher frequencies within the radio frequency (RF) range. As a result, parametric frequency multipliers are often connected to power sources generating the desired power levels at lower frequencies. Such frequency multipliers typically incorporate a varactor diode to multiply the lower frequency power signal to the higher frequency signal eventually required by the particular application.
Although a varactor diode is a diode, the process of frequency multiplication is not the result of rectification. Instead, frequency multiplication is accomplished by the voltage dependent nonlinear junction capacitance of a varactor diode. Because this property is reactive, it is possible to achieve high efficiencies.
A basic explanation of the frequency multiplication process is that an AC voltage causes the junction capacitance of the diode to vary nonlinearly throughout each cycle of the fundamental (or "pump") frequency. This is generally equivalent to periodically pulling apart the plates of a parallel plate capacitor in order to rhythmically change the capacitance. Whenever the capacitor plates have a voltage across them, the mechanical work involved in separating the plates is translated into increased electrical field energy. Similarly, relaxing the force results in translation of the stored electrical field energy back to mechanical energy. If, this "pumped" capacitor is placed in a location common to two resonant loops and the resonances of the loops are harmonically related, it is possible to transfer energy from one loop to the other and hence from one frequency to the other.
A persistent problem with frequency multiplier circuits has been that the varactor diodes themselves have presented a significant power limitation. The following factors have contributed to this power limitation: (1) the breakdown voltage of the diode; (2) the operating temperature limit of the diode; and (3) the efficiency of the diode. Varactor diodes utilized in the most demanding applications have typically been constructed of gallium arsenide (GaAs), which offers better operating characteristics when compared with a similar silicon (Si) diode. Even GaAs diodes, however, are generally limited to operation temperatures of less than 150.degree. C. and have breakdown voltage limits of about 15-25 volts for efficient operation. Higher power diode equivalents can be produced by connecting several GaAs diodes in series. A string of four such diodes would typically achieve a breakdown voltage of 50-60 V. With diode strings, however, it is difficult to design a heat sink structure compatible with microwave "plumbing".
Another problem with many prior art frequency multiplier circuits has been a relatively narrow bandwidth. A narrow bandwidth is, of course, acceptable so long as the frequency of the RF input signal is invariant. Many modern applications, such as phased array radar systems, require operation across a range of frequencies. For maximum power output over the range, it is necessary that any power circuitry utilized in the system, including the frequency multiplier circuitry, be inherently wideband. Additionally, a relatively wide bandwidth in the theoretical circuit design facilitates tolerance of undesired component variances which inevitably appear in physical realizations.